HFR technologies, such as the Spectral Band Replication (SBR) technology, allow you to significantly improve the coding efficiency of traditional perceptual audio codecs (referred to as core encoders/decoders). In combination with MPEG-4 Advanced Audio Coding (AAC), HFR forms a very efficient audio codec, which is in use, for example, within the XM Satellite Radio system and Digital Radio Mondiale, and also standardized within 3GPP, DVD Forum and others. One implementation of AAC with SBR is called Dolby Pulse. AAC with SBR is part of the MPEG-4 standard where it is referred to as the High Efficiency AAC Profile (HE-AAC). In general, HFR technology can be combined with any perceptual audio (core) codec in a back and forward compatible way, thus offering the possibility to upgrade already established broadcasting systems like the MPEG Layer-2 used in the Eureka DAB system. HFR methods can also be combined with speech codecs to allow wide band speech at ultra low bit rates.
The basic idea behind HFR is the observation that usually a strong correlation between the characteristics of the high frequency range of a signal and the characteristics of the low frequency range of the same signal is present. Thus, a good approximation for the representation of the original input high frequency range of a signal can be achieved by a signal transposition from the low frequency range to the high frequency range.
High Frequency Reconstruction can be performed in the time-domain or in the frequency domain, using a filter bank or a time domain to frequency domain transform. The process usually involves the step of creating a high frequency signal, and to subsequently shape the high frequency signal to approximate the spectral envelope of the original high frequency spectrum. The step of creating a high frequency signal may, for example, be based on single sideband modulation (SSB) where a sinusoid with frequency ω is mapped to a sinusoid with frequency ω+Δω where Δω is a fixed frequency shift. In other words, the high frequency signal (also referred to as the highband signal) may be generated from the low frequency signal (also referred to as the lowband signal) by a “copy-up” operation of low frequency subbands (also referred to as lowband subbands) to high frequency subbands (also referred to as highband subbands). A further approach to creating a high frequency signal may involve harmonic transposition of low frequency subbands. Harmonic transposition of order T is typically designed to map a sinusoid of frequency ω of the low frequency signal to a sinusoid with frequency Tω, with T>1, of the high frequency signal.
As indicated above, subsequent to creating a high frequency signal, the shape of the spectral envelope of the high frequency signal is adjusted in accordance to the spectral shape of the high frequency component of the original audio signal. For this purpose, scale factors for a plurality of scale factor bands may be transmitted from the audio encoder to the audio decoder. The present document addresses the technical problem of enabling the audio decoder to determine the scale factor bands (for which scale factors are provided from the audio encoder) in a computationally and bit rate efficient manner.